The "Broadband Integrated Services Digital Network" or B-ISDN is a new telecommunication technology developed by the telecommunication industry for both data transmission and telecommunications applications. B-ISDN is conceived as a carrier service to provide high speed communications to end users in an integrated way.
The technology selected to deliver the B-ISDN service is called "Asynchronous Transfer Mode" or ATM. The almost universal acceptance of ATM comes from the fact, that ATM handles a heterogeneous mix of communication traffic such as voice, data, image, video, and high quality sound. ATM can be used both in the LAN (Local Area Network) and the WAN (Wide Area Network) network environments and offers seamless interworking between the two environments.
ATM is effective in a much wider range of communications environments than any previous technology. However ATM is a compromise. ATM does not handle voice as efficiently (or as cost effectively) as does an isochronous network. ATM does not handle video as easily as isochronous transfer systems do (although ATM is more efficient). ATM certainly does not handle data as effectively or efficiently as a Packet Transfer Mode or Frame Relay system. ATM is likely to be problematic in any high error rate environment (such as some slower copper wire connections). Nevertheless ATM handles all types of traffic adequately and in an integrated way. This means that instead of having a proliferation of many specialised kinds of equipment for different functions it is now possible to have a single type of equipment and network which will do everything.
ATM technology is based on several key concepts. One key concept is that all information (voice, image, video, data . . . ) is transported through the network in very short, fixed length (48 data bytes plus a 5-byte header) blocks called "cells". The ATM cell size was determined by the Standardization Group CCITT (now called ITU) as a compromise between voice and data requirements. Another key concept is that ATM is designed so that simple hardware based logic elements may be employed at each node of a network to perform the switching. On a link of 1 Gbps, a new cell arrives and a cell is transmitted every 0.43 microseconds, leaving little time to decide what to do with an arriving cell.
Another key concept is that information flow is along paths called "Virtual Channels" (VC), which can be grouped into "Virtual Paths" (VP) established over physical links or lines and set up as a series of pointers through the network. Each cell header contains identifiers, called "Virtual Path Identifier" and "Virtual Channel Identifer" (VPI,VCI) that identify the connection that the cell belongs to and that allow the network nodes (sometimes identified as switches) to route the cell towards its destination. Cells on a particular Virtual Channel always follow the same path through the network and are delivered to the destination in the same order in which they are received.
A Virtual Channel Connection is a end-to-end logical connection along which a user sends data from a source to a destination. A multicast Virtual Channel Connection is a one-to-multiple logical connection along which a user sends data from a source to multiple destinations. Virtual Channel Connections may be established using many different types of media and need to be handled individually to guarantee that the traffic characteristics (such as rates, burstiness, quality of service) defined for the connection are met. This concept allows a cell which is to be multicasted to N destinations to be physically transported over less lines than the full N lines required in the previous systems.
Further information about ATM and other high speed networks can be found in a number of publications; for example, the International Teleconmmunication Union (ITU) Recommendations.
Generally, an ATM network is a set of ATM switch nodes connected and linked together by physical lines (carrier links) and used to route cells between ATM endpoints (source or destination endpoints). In the past, most of the switches were homogenous since the same transmission protocol was used throughout a network to transport the same kind of data. Private Branch Exchange P.B.X.) which handled the same format of information. More recently, the marketplace has required the development of heterogenous or multi-protocol switches which are able to handle multiple different protocols.
Basically, in a multi-protocol switch, input data lines must be connected to an adapter which includes a special device, a "protocol engine" which operates on data provided over the input data lines to package that data in a cell format needed by the switching elements. The switch node is designed to operate with a multitude of adapters, each of which converts input data from a particular format to the comment cell format employed within the switch.
In the emerging world of diverse communication traffic (multimedia, video distribution/conferencing) there is a need for a system having the capability of handling multipoint connections; that is, of sending the same cell to different end-users. Cells intended for multiple end-users are called multicast or broadcast cells. Generally, the source of a multicast cell does not have complete addressing information for each of the cell's multiple destinations. The multicasting function must be provided at the nodes of the network. This requirement has been taken into account in various switch implementations. A typical one is the so-called shared buffer type which is particularly attractive, and allows such an implementation with ease.
U.S. Pat. No. 5,394,397 from Yanagi et al. describes an ATM switching system which includes an input interface which is provided every incoming line and serves to convert header information of each input cell into internal routing information, a shared buffer memory and a cell writing control unit which forms normal cell list structures which are prepared in correspondence to outgoing lines and in which a plurality of normal cells are chained together with their next addresses. Cells to be multicasted are chained together with their next addresses in a broadcast cell list structure located within a switch device. A cell reading control unit includes a broadcast destination table and allows reading of cells from the broadcast cell list in lieu of normal cells. Although multicasted cells are simultaneously generated on the different outputs, one drawback of this system is that a cell cannot be sent several times on the same output.
U.S. Pat. No. 5,410,540 from Aiki et al. describes several embodiments based on different implementations of a shared-buffer-type ATM switch including a shared buffer memory, a multiplexer, a demultiplexer, a buffer memory controller and a cell copy section disposed between the multiplexer and the memory. The cell copy section produces a plurality of copies of a broadcast cell according to information in a copy information table and adds associate routing information to each copied cell so as to write the cells in the memory. In response to an indication from an output counter, the cells are read from the memory to be distributed to output ports, thereby implementing the broadcast function. Although this implementation improves the throughput of the ATM switch, the multicast cells are sent only once on the different output ports and the system does not guarantee that there will be no cell loss in case of bursty traffic associated with multicasts.
None the aforementioned prior art systems is capable of handling multiple protocols.